The present invention relates to an improved absorbent composition comprising an aqueous solution of piperazine and methyldiethanolamine for the removal of acid gases, such as CO2, H2S and COS, from gas streams.
Gas streams from natural gas wells, refineries and the like typically contain undesirable acid gases such as CO2, H2S, COS, and mercaptans. Aqueous amine solutions are known to be useful for the absorption of acid gases from these types of gas streams. Typically an acid gas-rich gas stream is contacted with an aqueous absorbent composition comprising an amine solution at low temperature or high pressure in an absorber. The aqueous amine solution usually contains an alkanolamine such as triethanolamine (TEA), methyldiethanolamine (MDEA), diethanolamine (DEA), monoethanolamine (MEA), diisopropanolamine (DIPA) or hydroxyaminoethyl ether (DGA). Once the acid gases are removed from the gas stream, it is ready for sale or additional downstream conditioning. The acid gas rich solution is regenerated for reuse by driving off the absorbed gases in specialized equipment such as flash tanks and/or stripper columns typically operated at high temperature or low pressure.
Workers in the art have found that certain compounds, such as piperazine and monomethylethanolamine (MMEA) may be advantageously added to aqueous amine solutions to enhance their absorption capabilities. For example, U.S. Pat. No. 4,336,233 to Appl et al. discloses a process for removing CO2, H2S and in some cases COS from gases that contain these compounds by washing the gases with aqueous solutions containing MDEA and piperazine as an absorption accelerator. The amount of piperazine stated to be useful in these absorbent compositions is up to 0.8 moles per liter of aqueous solution. The ""233 patent also discloses that it is particularly preferred to add from 0.05 to 0.4, especially from 0.2 to 0.4, moles piperazine per liter of aqueous solution.
U.S. Pat. No. 4,997,630 to Wagner et al. discloses a staged process for the removal of CO2 and/or H2S using an aqueous absorption liquid containing MDEA. The aqueous absorption liquid may additionally contain from 0.05 to 1, preferably 0.1 to 0.8, mole per liter piperazine.
In contrast with the teachings of the prior art, applicants have discovered that improved absorbent compositions for the removal of acid gases from gas streams instead comprise aqueous MDEA/piperazine solutions containing greater than 1 mole piperazine per liter of aqueous solution. In particular, aqueous solutions containing greater than 1 mole piperazine per liter of aqueous solution and about 1.5 to about 6 moles MDEA per liter of aqueous solution have been found to provide superior absorption capabilities over aqueous MDEA/piperazine solutions containing 1 mole or less piperazine per liter of aqueous solution. The present MDEA/piperazine absorbent compositions advantageously provide equivalent CO2 removal at reduced absorbent composition circulation rates in the absorber and increased absorber temperature, as well as improved performance of downstream regeneration equipment.
The invention provides an absorbent composition for the removal of acid gases from gas streams, comprising an aqueous solution comprising: 1) greater than 1 mole piperazine per liter of aqueous solution; and 2) about 1.5 to about 6 mole methyldiethanolamine per liter of aqueous solution.
The invention further provides a process for the removal of acid gases from a gas stream. A gas stream containing an acid gas contacts the above absorbent composition to produce a sweet gas stream and an acid gas-rich absorbent composition stream.